In recent years, servers are consolidated at a data center or a center site and vigorously used by multiple sites through a wide area network for the purpose of server management and operation efficiency enhancement. A typical example is data aggregation. Data aggregation is a process in which server data at a center site is used by clients at various sites. In data aggregation, the clients at various sites access server data through a wide area network. Data access through such a wide area network requires longer data access time than data access through an intra-site local area network (LAN) due to increased delay time and decreased effective bandwidth. An increase in data access time poses a problem because it may decrease the efficiency of a data handling process.
A data access pattern in data aggregation varies with target data and access timing. For example, the apparatus management record of a particular site may be frequently read or written from the site. Meanwhile, the map of a site is not frequently written but is read by many sites.
When data is to be accessed, metadata is required in addition to the data to be accessed by a client. The metadata is the information about the data, such as the name and type of the data. More specifically, the metadata includes, for instance, the summary of real data (checksum), write time, and access authority. The data to be accessed by a client is referred to as the real data in order to differentiate it from the metadata.
A cache technology is used as a technology for increasing the speed at which server data is accessed through a wide area network. The cache technology stores a copy of real data in a cache near a client and uses the cached real data to suppress a wide area network communication during data access. An agent that stores real data in a cache at a site is hereinafter referred to as a node. When the node is about to use real data stored in a cache through the use of the cache technology, it is necessary, from the viewpoint, for instance, of recency and access right, to perform an availability judgment process to determine whether the real data is available. This process uses the metadata.
When the cache technology is used, the metadata is generally managed by a server. One reason is that the metadata is smaller in size and transferred in shorter time than the real data. Another reason is that when the metadata is stored in multiple nodes, it is necessary to exercise complex control and determine whether the metadata stored in the server is up to date. However, when the metadata is managed by the server, each node has to access the metadata in the server through a wide area network in order to perform the availability judgment process. When the metadata is accessed, the effective bandwidth are not significantly decreased by the use of the wide area network because the metadata is small in size. However, accessing the metadata takes tens of milliseconds to several seconds due to an increase in delay time. As accessing the cached real data takes only several milliseconds to tens of milliseconds, it is preferred that the availability judgment process be completed without accessing the server.
A related technology disclosed, for instance, in US Patent Publication No. 2007/0162462 allows a node accessing particular data to manage metadata in accordance with access frequency. The related technology handles a collection of data as a file and uses a cache to increase the speed of file access. The related technology regards a node that frequently accesses a certain file as a home, and allows the home to cache real data and metadata of the file and manage the metadata. Nodes other than the home cache real data and make an inquiry to the home about the metadata when a file is to be opened to read. The home makes a response by using the managed metadata and manages, in accordance with the inquiry, the fact that another node uses the cache until the file reading is closed. At first, only the home has the authority to write data. This authority is referred to as the owner authority. When the data is to be written, the home notifies a node using the cache that the cache becomes invalid. When a node other than the home performs a write, the owner authority is temporarily transferred to the node that performs the write. The node to which the owner authority is transferred performs the write within a time limit, notifies the home of the write, and returns the owner authority to the home. The related technology allows a node that frequently accesses a file to store the real data and manage the metadata, and increases the file access speed by performing the availability judgment process without communicating through the wide area network when the node accesses the file.